Civil and geotechnical engineers can readily appreciate the importance of permeability in the design of groundwater lowering systems – permeability is a traditional numerical engineering value. However, engineers are some-times less proficient in recognizing the less quantifiable effects that aquifer boundary conditions have on groundwater flow to excavations. The fol-lowing sections will outline some important aquifer boundary conditions.
3.6.1 Interaction between aquifers and surface water
It is obvious from the hydrological cycle (see Section 3.1) that groundwater is inextricably linked with surface waters such as rivers, streams and lakes.
The significance of the link between groundwater and surface water at a given site depends on the geological and hydrological setting.
Where surface water flows across or sits on top of an aquifer, water will flow from one to the other – the direction of flow will depend on the relative hydraulic head. The magnitude of the flow will be controlled by Darcy’s law, and will be affected by the permeability and thickness of any bed sediment, and the head difference between the aquifer and the surface water. Bodies of surface water that are quiet or slow flowing may have low permeability bed sediments, which may dramatically reduce flow between surface and ground-water. Similarly, if the surface water is sitting on an aquitard or aquiclude it may be effectively isolated from groundwater in underlying aquifers.
Where watercourses (such as rivers and streams) are connected to aquifers they commonly receive water from the aquifer (the water entering the river from groundwater is termed ‘baseflow’). Such a river is said to be a ‘gaining’ river (Fig. 3.18(a)). This is perhaps contrary to many people’s expectations, that rivers should feed groundwater, rather than vice versa.
‘Losing’ rivers do sometimes exist, especially in unconfined aquifers with
deep water tables (Fig. 3.18(b)). If groundwater lowering is carried out near a ‘gaining’ river the hydraulic gradients may be reversed, changing a normally
‘gaining’ river to a ‘losing’ one. Even if the hydraulic gradients do not reverse, groundwater lowering may reduce baseflow to a gaining river. If pumping continues for an extended period this may reduce river flow, and perhaps result in environmental concerns.
Lakes often have low permeability silt beds, reducing the link with groundwater, although wave action near the shore may remove sediment, allowing increased flow into or out of the groundwater body. Man-made lagoons or dock structures may have silt beds (especially if they are rela-tively old) or may have linings or walls of some sort – however, just because linings exist, it does not mean that they do not leak! Analysis of a pumping test (Section 6.6) can be an effective way of determining whether groundwater low-ering will be significantly influenced by any local bodies of surface water.
3.6.2 Interaction between aquifers
In the same way that there can be flow between groundwater and surface water, groundwater can flow between aquifers if hydraulic head differences exist between them.
Many aquifer systems exist under ‘hydrostatic conditions’ – this is the case when the hydraulic head is constant with depth, so that observation Figure 3.18 Interaction between rivers and aquifers. (a) Cross-section through a
gaining river, (b) cross-section through a losing river.
wells installed at different depths show the same level (Fig. 3.19(a)). In this case there would be no flow of groundwater between shallow and deep aquifers separated by an aquitard. But sometimes non-hydrostatic condi-tions exist. In Fig. 3.19(b), the hydraulic head in the deep aquifer is greater than the shallow aquifer, so water will flow upwards from the deep to the shallow aquifer. The flow may be very slow if the aquitard is of low perme-ability; if the stratum between the aquifers is an aquiclude, the flow will be so small that it is often ignored in the analysis of short-term groundwater lowering installations.
Figure 3.19 Interaction between aquifers. (a) hydrostatic conditions, (b) flow between aquifers.
Inter-aquifer flow may be a long-term phenomenon, perhaps sustained by different recharge sources for each aquifer. On the other hand, it may be a short-term condition resulting from temporary groundwater lowering oper-ations disturbing the groundwater regime (this artificially induced flow between aquifers is one possible side effect of dewatering, see Section 13.3).
3.6.3 Recharge boundaries
Zones or features where water can flow into an aquifer are termed
‘recharge boundaries’, some commonly occurring examples of which are shown in Fig. 3.20. If recharge boundaries exist within the distance of influ-ence they can have a significant effect on the behaviour of dewatering schemes. They may cause the cone of depression to become asymmetric (since the extent of the cone will be curtailed where it meets the recharge source). The flow rate that must be pumped by a dewatering system will
Figure 3.20 Potential aquifer recharge boundaries.
often be increased by the presence of a recharge boundary. It is essential that any potential recharge boundaries be considered during the investiga-tion and design of dewatering works.
3.6.4 Barrier boundaries
Real aquifers are rarely of infinite extent and may be bounded by features which form barriers to groundwater flow. Fig. 3.21 shows some commonly occurring barrier boundaries. The presence of barrier boundaries will tend to reduce the pumped flow rate necessary to achieve the required drawdown.
3.6.5 Discharge boundaries
Water can sometimes be discharged naturally from aquifers. Water will flow from an unconfined aquifer if the water table intersects the ground surface. Diffuse discharges are called seepages or, if the flow is very local-ized (perhaps at a fault or fissure), the discharge is termed a spring. Flowing artesian aquifers can also discharge if faults or fissures allow water a path to the surface. Water flowing between aquifers may also constitute a dis-charge boundary condition.
Man’s influence, in the form of pumping from wells (either for supply or for groundwater lowering for construction, quarrying, mining, etc.) can also create discharge boundaries.
Fig. 3.22 shows some examples of discharge boundaries.
Figure 3.21 Potential aquifer barrier boundaries.